1. Field of the Invention
The present invention relates to techniques for processing photoresist materials generally used in the fabrication of microelectronic, micro-optical and micromechanical devices.
2. Description of the Background Art
Various types of microelectronic, micro-optical, and/or micromechanical structures may exhibit overall dimensions ranging between 50 microns (μm) to a few millimeters (mm). Such structures are useful in creating devices that are purely mechanical (such as watch gears), electro-mechanical (such as electrostatically-driven vibrating elements), optical (such as arrays of lenses), electro-optical (such as movable micro-mirrors), or for use with fluids (such as ink-jet printing heads).
A common method for fabricating such structures utilizes positive photoresist, which is applied in a thin layer to a substrate. Following application to the substrate, the free surface of the positive photoresist is positioned under a mask that is opaque in some regions and transparent in other regions. The positive photoresist is subsequently exposed on its free surface to ultraviolet (UV) light, which is patterned by passage through the mask. Positive photoresist is softened by exposure to UV light, and when the exposed photoresist layer is subsequently developed by rinsing in a developing solution, the UV-exposed and softened regions dissolve in the developer and wash away, leaving the unexposed photoresist in place on the substrate.
Typically, the positive photoresist layer is less than 100 μm in thickness, and in most case, the entire thickness of this layer in the UV-exposed regions is removed by the developer, yielding photoresist structures that have nearly vertical side walls. It is typical that the side of the substrate upon which the photoresist resides is then subjected to an etching process that transfers the pattern in the photoresist to the underlying substrate. The substrate is shielded from the etching process in the regions in which photoresist not exposed to the UV light still remains on the substrate. By this sequence of steps, and by repeated application of this sequence of steps, structures with complex shapes may be fashioned in the substrate. One may use this sequence of steps to fabricate micro-structures having minutely stepped surfaces, for example.
If a continuously curved photoresist structure is required, one may expose positive photoresist on its free surface through a mask that permits varying doses of UV light to penetrate the mask and illuminate the photoresist. The mask may be a gray-tone mask, in which different areas contain different UV light transmission fractions; or it may be a mask having very small low UV transmission dots of varying sizes or dot densities, selectively placed in regions of otherwise high UV light transmission. Regions of the positive photoresist exposed to a high enough dose of UV light will soften throughout the thickness of the photoresist, as described above. Regions of photoresist exposed to lesser doses will soften from the exposed free surface to diminished depths, depending on the UV light dose. Upon development, the positive photoresist remaining on the substrate will exhibit variations in thickness, corresponding to the variations in UV dose that the mask allowed to penetrate to the photoresist layer. This pattern of varying thickness of positive photoresist can then be transferred to the substrate by known dry etching techniques.
A major limitation results from the fact that the greatest thickness of positive photoresist that may be processed in the manner described above is generally less than 100 μm. This is a result of the high absorption of UV light in positive photoresist. If one attempts to expose too thick a layer, the UV light fails to adequately penetrate the deeper-lying volumes of the positive photoresist. As a result, the deeper-lying volumes fail to soften, therefore undesirably preventing full-thickness removal of the resist.
For some applications, however, it is advantageous to fabricate or fashion photoresist structures having thicknesses that are much greater than those attainable using positive photoresist.
A very useful class of photo-sensitized epoxy resists has been developed which has been shown to be useful at resist thicknesses up to 2 mm. An example of this class of resists is SU-8, currently manufactured by MicroChem Corp. of Newton, Mass., and by Sotec Microsystems SA, of Renens, Switzerland. This resist is a negative photoresist; in contrast to the behavior of positive photoresist, SU-8 toughens by polymerization upon proper exposure to UV light (of wavelengths near 365 nm). An attractive feature of this class of materials is its ability to produce structures with almost vertical side walls with thicknesses as great as 2 mm, which is much greater than that of any other photoresist.
All available literature that discusses the processing of this material is directed at teaching the best sequence of steps and parameters of individual process steps to improve the user's ability to fabricate structures in the photoresist having the full height of the film and nearly vertical side walls. Although the material can be spun on a substrate at a variety of initial thicknesses, all structures then fabricated in that layer are generally expected to have essentially the same thickness as that of the initial film.
In order to achieve structures in the photoresist having varying thicknesses, a step approximation to smoothly curved surfaces can be fabricated using a technique in which several layers of the photoresist are applied in succession, each one being processed individually. This technique can be used for fabricating a final structure containing steps, each step corresponding to the thickness of one of the several layers making up the final structure. In this technique, again, one finds that the steps are nearly vertical.
A well-accepted sequence of process steps for fabricating structures having the full height of the photoresist film with nearly-vertical side walls in a layer of SU-8 is as follows:    1. Clean the substrate and apply an adhesion-promoter like hexamethyidisilazane (HMDS).
Subsequent steps are done in a room in which green, blue and UV light are excluded (orange room), since SU-8 is sensitive to short wavelength visible and near ultraviolet light.    2. Spin-on a desired thickness of SU-8. The starting material is a viscous liquid mixture of SU-8 resin (typically bisphenol A novolac glycidyl ether), a solvent for SU-8 such as γ-Butyrolactone (GBL) or propylene glycol methyl ether acetate (PGMEA), and a photo-acid generator such as a triaryl sulfonium salt (e.g. Cyracure UVI, Union Carbide Corp.). Varying the ratio of resin to solvent yields mixtures with different viscosities at room temperature. One selects the mixture that will permit spinning the layer of desired thickness at spin speeds between approximately 500 and 5000 rpm.    3. The substrate with its spun-on layer is permitted to rest on a level surface so that the viscous SU-8 layer can flatten, and then the substrate is placed on a hotplate with final temperatures in the neighborhood of 950 C (softbake). The step causes evaporation of the solvent from the layer. As the solvent evaporates, the SU-8 layer that remains on the substrate becomes more viscous. However, it remains a liquid at 95° C. since even pure SU-8 has a glass transition temperature of approximately 55° C.    4. When the solvent has evaporated from the spun-on layer, the substrate is removed from the hotplate and cooled to room temperature. At room temperature, the SU-8 layer is a solid.    5. The SU-8 layer is exposed with light whose wavelengths are between 300 and 400 nm. The light is patterned on the SU-8 layer by the use of a mask which has areas that are opaque to the exposing illumination as well as areas that are transparent. This process is typically carried out in a mask aligner, and the mask may be first positioned to align with structures already on the substrate. In this step, the exposure is made from the SU-8 side of the substrate. The areas of the SU-8 that are exposed to this light release photo-acid from the photo-acid generator which causes the SU-8 resin to crosslink. An important consideration in selecting the energy per area (dose) of the exposure is to assure that the entire film thickness of exposed areas will polymerize completely.    6. The substrate with its exposed SU-8 layer is placed on a hotplate, with final temperatures of at least 95° C. This step greatly accelerates the cross-linking of the SU-8 material in the areas exposed to the UV light. In the ideal case, the entire thickness of SU-8 material in exposed areas will become fully polymerized. The material in the unexposed areas remains unpolymerized.    7. The patterned SU-8 layer is developed. The standard method involves placing the substrate with its exposed SU-8 film in a bath or in a sequence of baths containing a solvent for unpolymerized SU-8. In these baths, the unexposed and unpolymerized areas of the SU-8 film are dissolved away and ideally only the polymerized areas remain attached to the substrate.    8. The substrate with its developed fully-polymerized SU-8 structures is then rinsed and dried.
Methods for creating structures with smoothly-varying thicknesses in this class of materials do not exist. If processes designed for creating continuously curved surfaces in positive photoresist were applied to this material, such as by exposing the SU-8 on its free surface through a mask that permits varying doses of UV light to penetrate the mask and expose the photoresist, the SU-8 would polymerize first near the free surface if the dose there were sufficient., At greater depths into the photoresist film, the SU-8 might not polymerize. With development, the unpolymerized volumes of SU-8 near the photoresist-substrate interface would dissolve, and the entire SU-8 film would undesirably lift off the substrate.
What is needed is a method for fabricating structures in this and related classes of materials, where such structures may be characterized by thicknesses that vary smoothly with position. The method should minimize the number of masking and other processing sequences required to fabricate such structures using a film of suitable initial thickness.